The present invention relates generally to wireless communications, and more particularly to a method and apparatus for effecting a handoff in a cellular communication system.
Cellular communications systems usually include several base transceiver stations that transmit and receive channels of information throughout a predetermined area referred to as a cell. The base transceiver stations are typically arranged to provide continuous service to mobile subscribers throughout a particular geographic region.
When a mobile subscriber is engaged in a call, the mobile subscriber will frequently approach the boundary between two cells. As the mobile subscriber nears the boundary of the serving cell, the signal strength of the traffic channel from the mobile subscriber will begin to fall in the serving cell. When the signal strength falls below a predetermined threshold value, the base transceiver station of the serving cell requests the base station controller to determine whether the mobile signal equals or exceeds a predetermined threshold value in one of the neighboring cells. If a neighboring base transceiver station detects an adequate signal level of the mobile subscriber, a handoff will be initiated in order to transfer the call to a selected traffic channel of the neighboring cell.
In conventional digital TDMA cellular systems, a handoff of a mobile subscriber usually requires the base transceiver station of the serving cell to transmit a signal to the mobile subscriber commanding it to transmit several commands in shortened bursts to a large neighboring cell. The shortened bursts contain information that allows the neighboring cell to determine initial synchronization including the initial timing offset. However, the transmission of the shortened bursts adds to the communication overhead of the system by consuming valuable transmission time. In particular, the time spent sending the shortened bursts over the communication channel could be used to transmit other information, such as telephony data. In addition, the transmission of shortened bursts may result in queuing delays and system congestion, causing significant speech interruption. These problems are magnified when one considers that typical cellular systems usually service numerous mobile subscribers.
Accordingly, there is a need for an improved apparatus and method for performing a handoff in a digital TDMA wireless communication system. It would be desirable for the apparatus and method to decrease the amount of time needed to perform a handoff. It would also be beneficial to provide an apparatus and method that can reduce the amount of transmissions needed during a handoff.